The present invention relates to a color filter, and a liquid crystal display device having the same. In particular, the present invention relates to a color filter having colored layers containing a dye-bonded polymer, and a liquid crystal display device having the same.
As a color filter for a display device, various types are known, but a color filter for liquid crystal display is herein explained as a typical example. Various liquid crystal displays are widely used, including from liquid crystal displays having a small area for, e.g., a clock and a camera to ones having a large area for, e.g., a computer terminal display device and a television screen display device. Recently, color display has rapidly been developed mainly for liquid crystal display devices having a large area. A color filter is essential for colored liquid crystal display, and is an important member which decides the performance of color liquid crystal display. In order to display highly minute images, color filters which are minutely patterned into various forms are used. As a conventional method for producing a color filter, there are known dyeing, ink jet, printing and photolithographic methods.
The dyeing method is a technique of producing a color filter by repeating, for each of red, green and blue colors, the step of forming a given filter pattern and then dyeing the filter pattern in each of the colors and the step of blocking the dyeing. This method has such drawbacks that the steps are complicated and the number of the total steps is large. Against this method, proposed are color filter producing methods that have fewer steps and are more effective. These methods include, for example, the ink jet method that uses an ink jet device, wherein an ink is jetted onto a transparent substrate, as a base, and the ink on the substrate is fixed to color the substrate; and a printing method that uses a printer to transfer an ink onto a transparent substrate as a base. In these methods, the positioning of patterns is performed by means of an ink jet head, or a printer and a printing plate. Therefore, the step of patterning colored layers is unnecessary. According to these methods, however, it is difficult to form a highly minute filter.
The photolithographic method is classified into direct photolithography and indirect photolithography (the etching method). The direct photolithography is to apply a color solution wherein a colorant and a photosensitive polymer are dissolved or dispersed in a solvent onto a substrate to form a colored layer, irradiate (pattern) the colored layer with light, and develop the colored layer to form a one-colored pattern. In this method, these consecutive steps are repeated for respective colors in the same way as in the dyeing method. However, this method does not require any dyeing step or the step of blocking the dyeing. Therefore, the steps in the direct photolithography are fewer than those in the dyeing manner. Besides, it is possible to realize control of spectroscopic property and reproducibility easily. Since photolithography is used in this method, resolving power is high so that a highly minute color filter, which cannot easily be obtained by the printing method or the ink jet method, can be made. Recently, therefore, this method has been the main current in the production of color filters for liquid crystal display.
The etching method is to apply a photoresist having photosensitivity onto a colored layer having no photosensitivity, dry the photoresist, irradiate the photoresist with light to form a pattern, conduct treatment with a solvent with which only the colored layer can be removed, and remove the photoresist. These consecutive steps are repeated to make a color filter. The etching method is a promising method for forming highly minute pixels.
In such methods using photolithography, a pigment has been conventionally used as a colorant since the pigment generally has good resistances against heat, light and chemicals. However, the pigment has a certain particle size bringing about depolarization, and thus it is known that the display contrast ratio of color liquid crystal devices deteriorates. It is also difficult to obtain high light transmissivity. Thus, such methods have a limit for an improvement in brightness of color filters.
On the other hand, dyes are generally soluble in solvents or polymers. Further, dyes are not aggregated in colored solutions for color filters, and are stable. In a color filter obtained by using a colored solution wherein a dye is dispersed, depolarization does not arise since a dye is dispersed at a molecule level. Light transmissivity is also good. However, dye-dispersed color filters have poorer resistances against heat, light and chemicals than pigment-dispersed color filters.
In order to improve the resistances against heat, light and chemicals, for example, the following methods are proposed: a method that uses a colored solution for color filters wherein a polyimide precursor, a disperse dye and an oil-soluble dye are dissolved in a solvent, so as to form a pattern by the etching method (Jpn. Pat. Appln. KOKOKU Publication No. 4-243), and a method that uses a colored solution for color filters comprising a positive resist, a crosslinking agent and a dye to form a pattern by the etching method (Jpn. Pat. Appln. KOKAI Publication No. 4-301802 and Jpn. Pat. Appln. KOKAI Publication No. 6-35183). In all of these methods, colored compositions wherein a dye, instead of a pigment, is merely dispersed in a resin are used. There remains a problem that the dye is eluted out in development or the dye is sublimated in heating.
Concerning some of the dyes, in order to solve the problems of elution and sublimation, it has been proposed to covalently bond an acryloyl group to a dye and copolymerize the resultant dye with other polymerizable monomer. The proposed method is, however, related to only specific dyes in academic study. There is not any specific description on common dyes. The method is not proposed for color filters. As a result, it is not considered that some hue is changed at the time of introducing a substituent such as an acryloyl group.
Thus, by photolithography there is not obtained any practical color filter wherein a dye is used to have high contrast ratio, minuteness, transparency and color purity and excellent resistances against heat, light, chemicals and sublimation.
Therefore, an object of the present invention is to provide a dye-containing color filter, making it possible to prevent a dye from being sublimated, having higher spectroscopic property and contrast ratio than color filters using only a pigment, and exhibit high resistances against heat, light and chemicals.
The above object and other objects which will be evident from the following description have been achieved according to the present invention by a color filter comprising a plurality of colored layers which are arranged adjacently to each other and have different spectroscopic properties, wherein at least one of the plurality of colored layers contains a dye chemically bonded to a polymer chain.
In the present invention, the dye bonded to the polymer chain may be an oil-soluble dye such as an anthraquinone or triphenylmethane dye.
According to the present invention, there is also provided a liquid crystal display device comprising a color filter of the present invention.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.